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SERVICEMAN'S LOG
When I switch it on, nothing happens
The old gag about the power switch being at
fault because the set won’t work when you turn
it on is taking on a new twist these days. And it’s
no longer a gag – these days, when the remote
control system fails, the set won’t work.
That was the situation I faced recently, involving a Superstar brand
remote control colour TV set – 34cm
model 1401R made in China. It was
another repair for a colleague, so I
had only a secondhand version of
the fault. But the complaint was
straightforward enough; the set was
completely dead.
This is one of those sets which
can only be switched on or off by
the remote control and that was the
first problem. The set came in with
the remote control but this was in a
rather grotty state. It had obviously
had a hard life, judging by its external appearance, and was even worse
inside.
For starters, the batteries were flat.
And although they hadn’t leaked, a
previous set of batteries had, as was
all too obvious from the badly corroded contacts. In fact, the corrosion
was so bad that one of the contacts
broke off as I was removing the dead
batteries.
The next problem was that I didn’t
have a circuit or manual and so I had
to track down the agents to get one.
And when I did finally get a manual,
the circuit turned out to about the
worst quality copy I have ever encountered. I would defy anyone to
decipher any of the values at anything
more than a guesstimation level – and
then only by cross referencing to the
set itself.
This not an unusual state of affairs
these days, unfortunately. I don’t know
who is to blame but I do know that
the service industry is being given a
pretty raw deal.
Anyway, back to the set itself. One
of the first things to determine in situations like this is whether there is a
fault in the set itself or a fault in the
remote control system. Initially, I set
about familiarising myself with the
layout and making some preliminary
checks which might suggest where
the fault lay.
And, in order that the reader can follow the story, it is necessary to convey
some idea of the circuitry – something
which is made all the more difficult
by reason of the poor circuit quality
which I’ve already mentioned (the
only other justification for reprinting
it would be as an ‘orrible example).
Voltage checks
My first step was to identify and
check the main voltage rails. As far as
I could see, there were three: a 120V
rail and two 12V rails, which I will
call “A” and “B”.
The 120V rail is derived from a
switchmode power supply. This
involves the usual bridge rectifier
(D121) across the mains, a chopper
transformer (EM110), and IC104,
which provides the oscillator and
Fig.1: the power supply circuitry in the Superstar 1401R TV receiver. The bridge rectifier is
at lower left, the chopper components (EM110 & IC104) centre and right, and transformer
EM112 above the bridge rectifier. Connector CN201 is at top left.
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The output from D121 is around 340V
and this is smoothed by a 100µF 400V
capacitor (C101). This is applied to
pin 1 of IC104 via the primary winding of EM110 (pins 2 & 4). The 120V
rail comes off pin 9.
The 12V “A” rail is derived from another winding on transformer EM110
(pins 12 & 13), via diode D108 and
voltage regulator IC103.
The 12V “B” rail, on the other hand,
comes from a small 50Hz power transformer (EM112), via diode D107 and
two filter capacitors (C127 & C128).
It is used to power the remote control
receiver and its associated circuitry,
ensuring that this is functional at all
times, even when the main part of the
set is shut down.
In general terms, this is all fairly
conventional. More importantly, it enabled me to make the first assessment
as to the broad nature of the fault. At
first switch-on, there was no 120V rail
and no 12V “A” rail. However, there
was output from the bridge rectifier
and there was 12V on the “B” rail. In
other words, the switchmode supply
wasn’t working.
The switchmode supply is turned
on and off – from the remote control
board – via a chain of three transistors: Q116, Q117 and Q118. In simple
terms, to turn the set on, a positive
voltage is applied to Q116’s base from
the remote control board (via pin 4
of plug/socket CN201). This turns on
Q116 which then turns off Q117 and
Q118.
Q118 is connected between pins 2 &
4 of IC104. From this, it appears that
the set is held off by connecting pins
2 & 4 together via Q118, when this is
turned on.
Conversely, when this transistor
turns off, the set turns on. And since
there was no positive voltage applied
to Q116’s base when the Power button
on the remote control was pressed, it
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April 1996 39
Serviceman’s Log – continued
was obvious that the set could not turn
on – quite apart from any other reason
why it may not work.
I pulled a swifty here – I set the
analog multimeter switch to the low
ohms range and connected the positive
probe to the chassis and the negative
one to Q116’s base. Like most such
meters, mine applies reverse voltages
to the probes when in the ohms range,
which meant that I was applying a
positive voltage to the base of the
transistor.
And it worked; the set burst into
life. Well, that was a major step
forward. The fault was quite clearly
in some part of the remote control
system. I still had to find out where
but the search had been narrowed
considerably.
Remote control section
The remote control section consists
of a photo receiver module, two ICs
(IC1 & IC3), a few transistors, and the
40 Silicon Chip
usual array of switches, diodes and
pots in the channel selection network.
Fig.2 shows part of this circuit.
At this point I had to get the remote
control unit itself working. Apart from
its grotty external appearance and the
broken battery contact, there wasn’t
a great deal wrong with it and I was
able to get it working on a temporary
basis. More permanent repairs could
come later.
The next thing was to determine
whether the photo receiver was
functioning. When a valid signal is
received, this should deliver pulses to
transistor Q4, which in turn drives pin
13 of IC3. In fact, the CRO confirmed
that all this was happening.
However, there was no positive
voltage produced at pin 6 of IC3,
which ultimately connects to the
base of Q116. And that seemed to
throw suspicion on either IC3 itself
or its associated circuitry. I checked
that 12V was being applied to pin 12
and that the clock crystal (Z1) was
functioning (the frequency meter
confirmed that this was oscillating
at 455kHz).
I made a few more checks of the
other associated parts but could find
nothing wrong. In short, it all came
back to the IC. I didn’t have a replacement, so I ordered a new one from the
agents (price $30 trade).
And while I waited for it, I tried
something else; I fitted a socket in
place of IC3. Now I know that sockets
have not enjoyed a very good reputation in the past and with good reason.
Some of the early attempts were pretty
woeful.
Fig.2: part of the remote control receiver in the Superstar 1401R. Q4 buffers signals from the photo receiver
module and drives pin 13 of IC3. The output from this IC appears at pin 6 and goes to pin 4 of connector CN201.
But the scene has changed for the
better and there are now some very
good quality units available. And
there is no doubt that a socket makes
things a lot easier where there may
be some doubt about the fault. On the
other hand, space around or above the
site often makes such a modification
impossible.
But there was room in this case,
so I went ahead. And as if to justify
what I had done, I suddenly found
a spare IC that I’d had all the time.
It wasn’t a new unit, having been
removed some time previously from
another set. Nevertheless, I pushed
it straight into the socket, switched
on and everything came good, with
all remote control functions fully
operative.
This not only confirmed that it was
the IC at fault but, in the process,
cleared everything else, including the
remote control unit itself.
I let the set run for the rest of the day
and all next day and it never missed
a beat. But on the third day it died.
I wasn’t particularly worried; the IC
was suspect, so I simply assumed that
it had failed and waited for the new
one to arrive.
When it did a couple of days later, I pushed it in and the set came
good again. I let it run as before but
took the opportunity to go over the
various adjustments and make sure
that everything was up to scratch. So
the job was virtually finished, or so I
thought until, a couple of days later,
the set suddenly died again.
Can something “die again”? Well
this set did and it came as a rude
shock. I had a horrible feeling that
there was a “nasty” lurking in there
somewhere, causing the set to fail
every few days.
A simple explanation
In fact, it was a simpler explanation
than that. A few meter checks revealed
that the 12V “B” rail had failed and
that this was due, in turn, to the failure of the EM112 transformer. In fact,
its primary winding measured open
circuit.
And that created a difficult situation. While I hadn’t quoted for the
job, I had given an estimate. A new
transformer would be expensive and,
when added to what had already been
chalked up, it wasn’t going to make a
very nice figure.
Then I had an idea – many of these
transformers feature an internal thermal fuse and I was prepared to bet long
odds that this was what had failed (it
wouldn’t be the first time).
So was it worth trying to fix? Well, I
didn’t have much to loose. The winding was wrapped in yellow plastic
tape and, armed with a razor blade, I
very carefully cut through it near the
winding terminals, where I judged it
was clear of the winding.
In fact it was and, working very
carefully, I was able to peel back
the tape to give a good view of the
winding. That was fine but Murphy
had seen to it that the thermal fuse
was on the opposite side to where I
had started. When I finally did reach
it, a quick check revealed that it was
open circuit.
The failure was not due to any normal fuse action; rather it appeared to
be a simple structural failure.
More to the point, what should I do
about it? In theory, I suppose, I should
have aimed to replace it. However,
I didn’t fancy the time and trauma
that would be involved in getting a
replacement.
Nor could I see the justification for it
in the first place. The set is adequately
fused in the mains lead, which should
surely take care of any fault which
could occur anywhere in the set. Why
pick on this component?
I simply bridged it, then rewrapped
the winding in new tape, refitted the
transformer and gave the set another
soak test. This lasted several days
and passed without further incident.
I handed the set back to my colleague,
filled him in on the thermal fuse situation, and left him to deal with his
customer. By all accounts, everyone
was satisfied.
Postscript: having done all the above
and written about it, I suddenly acquired another version of the circuit.
It is a quite different drawing but exactly the same circuit and, while not
perfect, a far better quality print (most
of it is readable). This is the one used
to illustrate this article.
The distorted Toshiba
My next story is about a Toshiba
48cm colour set, model 207E9A,
April 1996 41
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Fig.3: the vertical output stage of the Toshiba 147R9E. IC303 at left provides the vertical
output signal to the deflection yoke (note the input and output waveforms). Capacitor C317 is
at centre, while the deflection coils (L462) are at the extreme right.
made in Singapore, vintage 1989.
The complaint was gross vertical scan
distortion. Only the top half of the
screen had any recognisable image,
while the bottom half was compressed
in the centre.
A colleague has a theory about vertical distortion. His rule of thumb is
that if the problem is at the top of the
screen, it is a power supply problem;
if it is at the bottom, it is a feedback
problem.
Frankly, I’m always rather suspicious about general statements of that
nature but I have to agree that it has
some merit. Did it apply in this case?
I leave the reader to judge for himself.
The relevant sections of the circuit
involve two ICs: IC501 and IC303.
IC501 is a TA8718N, a 30-pin multi-purpose chip which provides most
of the front-end processing. This
includes colour decoding and the derivation of the vertical and horizontal
signals.
The vertical signal comes out on
pin 11 and goes to pin 4 of IC303
(AN5515). This is the vertical output
stage and the signal from pin 11 goes
into it on pin 4, comes out on pin 2,
and goes to terminal 7 of the vertical
deflection yoke.
My first step was to check the
voltages on IC303 and they came up
virtually spot on. Next, assuming that
it was a signal path fault, possibly in
the feedback network, I decided to
check out the various electrolytic capacitors, particularly the lower value
ones, which are notorious for poor
reliability.
And no sooner had I made that decision, than I found one staring me in
the face. It was a red Elna 2.2µF unit
(C317) in what appeared to be part
of the feedback path from terminal 8
of the yoke. It had leaked its inside
outside, all over the board around it.
Bingo, I thought. Picked it in one;
I’ll knock this one over in no time.
Alas it was not to be. I removed the
sick unit, cleaned up the board, fitted
a new one, and switched on. Result:
exactly as before.
Circuit waveforms
So it wasn’t going to be easy after all;
I would have to tackle it stage by stage.
The circuit shows two waveforms; the
input to IC303 on pin 4 and its output
on pin 2 – see Fig.3. I reached for the
CRO leads and checked pin 4. It was
virtually spot on, its amplitude and
shape exactly as shown.
But pin 2 was a different story. The
waveform was nothing like that on the
circuit. I followed the signal through
to the yoke (terminal 7) and then to
the other side of the yoke (terminal 8),
speculating on the remote possibility
of shorted turns in the yoke.
This check didn’t tell me much. For
some strange reason, the waveform on
terminal 8 was more like the circuit
pattern than the one direct from IC303
at terminal 7. If it meant anything at
all, it seemed to rule out the shorted
turns theory.
And that, in turn, put suspicion
back on IC303 and its sur
rounding
components. With one crook electro
already encountered, I first proceeded
to check all the electros around the
IC. And by checking, I really mean
replacing, because I felt this was the
only sure test when chasing a weird
fault like this one.
That achieved nothing. To cut a long
story short, I finished up checking or
replacing every component around
that IC – even the diodes. Nothing
made any difference, which left the IC
itself. It is a common type and I had
stock on hand so I changed it. Again
I drew a blank.
I was feeling pretty desperate by
April 1996 43
ning from terminal 8 of the yoke to pin
14 of this IC (via R304). And the circuit
indicates 6.7V on pin 14, which was
exactly what it measured.
Was the fault in IC501? I didn’t
fancy the time and expense involved
in changing this – I would have had
to order one – and looked around
desperately in this part of the circuit
for further inspiration. And I found it
in the most unexpected place.
Connected to the adjacent pin 13 of
IC501 is the height control (R351), a
50kΩ pot to chassis. Now I probably
would never have suspected this part
of the circuit in a month of Sundays
but what caught my eye was a bypass
capacitor, C303, from pin 13 to chassis
– it was a red Elna 2.2µF electrolytic,
identical to the one I had already replaced in the yoke circuit.
I should have spotted it sooner; it
was the only other red electro on the
board.
But having spotted it, I didn’t stop to
ponder the technical implications – I
reefed it out and replaced it. And that
was it; problem solved.
Still a mystery
now and came back to the idea of
a fault in the yoke winding. Not
surprisingly, I didn’t have another
yoke of that type on hand but I did
have a somewhat similar one from
another set. I decided to temporarily
substitute that, at least electrically,
and note whether it made any drastic
difference to the faulty waveform at
pin 2. It didn’t, so I finally ruled out
that theory.
So what was there left to check?
At this stage, I remembered my colleague’s theory about the feedback
circuit. I hadn’t consciously checked
this, as such, assuming that checking all obvious components would
include it.
But it hadn’t. The feedback circuit
also involves IC501, with a line run-
I’m still at a complete loss to explain
just how the height control came to
be involved in this particular fault.
But then, without knowing the exact
circuit details within the IC by which
the height is controlled, who can say.
Is the height control part of the feed
back circuit? And what is the function
of the 2.2µF capacitor which caused
the fault?
But those questions aside, the story reinforces what I’ve said so many
times before and with which all my
colleagues agree; never trust a low
SC
value electrolytic capacitor.
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